Organic thin film transistors (TFTs) are expected to become key components of plastic circuitry. TFTs have applicability as well to other systems such as silicon circuitry. One of the most significant challenges in bringing TFTs into commercial use is to facilitate the effective deposition of organic semiconductors on a substrate, preferably in a manner that is inexpensive as compared to silicon based technology. In order to exhibit suitable electrical properties, organic semiconductors preferably are deposited as thin and uniform films comprising relatively large single crystals within ordered polycrystalline domains, which is a difficult task. See, for example, C. Cai et al., “Self Assembly in Ultrahigh Vacuum: Growth of Organic Thin Film with a Stable In-Plane Directional Order,” J. Am. Chem. Soc., Vol. 120, p. 8563 (1998).
Much effort has been made to develop simple fabrication techniques for attaining the necessary uniformity and order in an organic semiconductor film. For example, several groups have experimented with solution casting of thiophene oligomer films, in which a solution of an organic semiconductor is essentially dropped onto a substrate and the solvent is evaporated by heating. However, these casting processes have generally provided relatively poor uniformity and coverage by such oligomeric thiophene solutes. More importantly, crystal sizes have typically been small and the resultant mobilities, even over small areas, have often been unacceptably low or non-uniform compared to films formed by vapor phase techniques. See, for example, A. Stabel and J. P. Rabe, “Scanning tunneling microscopy of alkylated oligothiophenes at interfaces with graphite,” Synthetic Metals, Vol. 67, p. 47 (1994); H. E. Katz et al., “Synthesis, Solubility, and Field-Effect Mobility of Elongated and Oxa-substituted α,ω-Dialkyl Thiophene Oligomers. Extension of ‘Polar Intermediate’ Synthetic Strategy and Solution Deposition on Transistor Substrates,” Chemistry of Materials, Vol. 10, No. 2, p. 633 (1998); H. Akimichi et al., “Field-effect transistors using alkyl substituted oligothiophenes,” Appl. Phys. Lett., Vol. 58, No. 14, p. 1500 (1991); M. Mushrush et al., “Easily Processable Phenylene-Thiophene-Based Organic Field-Effect Transistors and Solution-Fabricated Nonvolatile Transistor Memory Elements,” J. Am. Chem. Soc., Vol. 125, pp. 9414-9423 (2003); and Hong, X. M. et al., “Thiophene-Phenylene and Thiophene-Thiazole Oligomeric Semiconductors with High Field-Effect Transistor On/Off Ratios,” Chem. Mater., 13, pp. 4686-4691 (2001).
One process for forming devices utilizing organic semiconductor films, disclosed in Katz U.S. Pat. No. 6,403,397 issued on Jun. 11, 2002 and entitled “Process For Fabricating Organic Semiconductor Device Involving Selective Patterning,” involved treating a surface to selectively provide regions of greater affinity and lesser affinity for an organic semiconductor or an organic semiconductor solution. When the organic semiconductor, or solution comprising the semiconductor, was deposited on the treated surface, either the organic semiconductor or the organic semiconductor solution dewetted from the lesser affinity regions or the resultant film adhered only weakly to the lesser affinity regions such that selective removal was readily performed. Even where such removal was not performed, the portions of the organic semiconductor film overlying the greater affinity regions exhibited higher conductivity and better film continuity relative to the other portions of the film. Further processes for forming devices utilizing organic semiconductor films are disclosed in Katz et al., U.S. Pat. Nos. 6,265,243 and 6,551,717, respectively issued on Jul. 24, 2001 and Apr. 22, 2003, both entitled, “Process for Fabricating Organic Circuits.”
There remains a need for semiconductor devices comprising organic semiconductors in the form of larger and better ordered polycrystalline semiconductor domains having accordingly high channel mobility. There further is a need for methods of making such semiconductor devices.